1. Technical Field
The present invention relates to a method and an apparatus for manufacturing spark plugs used by an internal combustion engine mounted on an automobile.
2. Related Art
In an internal combustion engine, spark plugs are used to start the engine. The conventional spark plug is provided with a pillar-like center electrode held in an insulation manner within a housing member and an earth electrode coupled with one end of the earth electrode. The earth electrode is bent at its intermediate predetermined portion so as to make the tip thereof face one end of the center electrode with a space (called “spark gap”) formed therebetween.
A conventional method of manufacturing the spark plug is proposed by Japanese Patent Laid-open publication No. 2000-164320. This conventional method uses both of a provisional bending process and a main bending process to produce a spark plug. The earth electrode is first subjected to the provisional bending process, in which the spark gap is formed slightly larger than a specified size, and then subjected to the main bending process. In this main bending process, the spark gap is finely adjusted so that its size falls within a predetermined size range. In the case of this manufacturing method, before the main bending process, the tip surface of the central electrode is measured with regard to its position and other factors. Results from the measurement are used to determine an amount of bending of the earth electrode.
However, the bending technique proposed by the above reference results in that the other end of the earth electrode is positioned differently moment to moment on a plane perpendicular to an axial line axially passing the center electrode (, which is referred to as “a center-electrode axial line”). Thus, in the spark plug in which a noble metal chip is welded to the earth electrode, the position of the noble metal chip on the plane perpendicular to the center-electrode axial line is dependent on situations. Thus accuracy in the concentric factor between the center electrode and the noble metal chip is forced to be lowered, which is one reason of deteriorations in the ignitionability of a spark plug.